1. Field of the Invention
The present invention relates generally to fuel cells, and more particularly, the present invention relates to microbial fuel cells (MFCs).
2. Description of Related Art
Clean and efficient energy production from renewable resources is highly desirable due to the concurrent rapid increases in both energy demand and environmental concerns. Of particular interest are MFCs in which microorganisms act as biotransformation mechanisms, consuming oxidizable organic material containing chemical energy and producing useful energy in the form of electricity. Further, MFCs with enhanced power output have vast potential commercial applications. In general, MFCs are especially well suited for long term (months, years) and/or remote applications where refueling/recharging is not an option. However, the development of MFCs is in its infancy, and there is great potential to increase power output through electrochemical, microbiological, and systems engineering improvements.
The general definition of a fuel cell is a device that converts chemical energy to electrical energy. In the specific case of MFCs, the electrical energy is produced by microorganisms that metabolize organic materials. Microbes that are capable of such transformations are termed “electrogenic,” which refers to creating a net flow of charge. Electrons produced by the bacteria are transferred to an anode and then through an electrical circuit to a cathode.
Microorganisms are the power source within the MFC. In a mediator-less MFC, electrons produced by the metabolic activity of the microorganisms are transferred out of the microorganism to an electrically-conducting electrode. Microorganisms that are not in electrical contact with the electrode surface directly or via the electrically-conductive biofilm usually do not have access to an acceptor for the electrons generated, and as a result, these cells will not proliferate.
Current research in the area of MFCs has resulted in development of various MFC arrangements. However, such MFCs are generally unsatisfactory, producing low power density and low efficiency of conversion of a microbial nutrient fuel to useful energy, such that MFCs have so far been limited in production and application.
One strategy to increase MFC power output is to optimize the surface area and porosity of the anode material. It is advantageous to tailor these properties because electrical current correlates with both available surface area and density of electrogenic microorganisms. Increasing the surface area of the electrode can have a dramatic effect on the power output. For example, a fuel cell that contains a 10 cm3 three-dimensional electrode with a surface area of 100 cm2/cm3 has a theoretical power output one hundred times greater than one containing a 10 cm2 two-dimensional electrode (e.g., graphite rod, carbon cloth).
The porosity of the electrode material is also important because the structure must allow for the circulation of biological media (containing water, food/fuel, trace nutrients, etc.) to the entire microbial population. Electrogenic bacteria, such as Geobacter sulfurreducens and Geobacter metallireducens, are typically 1 micron in width and 1-2 microns in length, depending on the particular species. There must be room within the electrode structure for a biofilm to grow, as well as for fluid to move within the structure and allow fuel to diffuse into the structure and waste to diffuse away from the biofilm. If fuel and waste cannot be transported throughout the biofilm, the current production in different areas will vary and some cells will not be viable. For example, a biofilm of wild type Geobacter sulfurreducens or Geobacter metallireducens can generally grow to a depth of approximately 40 microns. Usage of microbes from other families of bacteria or microbes will provide biofilms of varying depth. Accordingly, the structural dimension of the structure should account and allow for the depth of the biofilm on its surface.
The following related art all utilize three-dimensional anode materials, but these materials are irregularly sized and shaped, which prevents the optimum growth and performance of electrogenic biofilms.
US patent publication 20070259217 “Materials and Configurations for Scalable Microbial Fuel Cells.” This patent publication discloses the utilization of a carbon fiber brush anode.
EP1742288(A1) [also EP1902489 (A2)] “Microbial fuel cells for oxidation of electron donors.” This patent discloses tubular, mushroom-shaped, and omega-shaped MFCs filled with conductive particles, namely graphite granules.
US patent publication 20060147763 (A1) “Upflow Microbial Fuel Cell.” This patent publication discloses a cylindrical anode chamber filled with granular activated carbon.
US patent publication 20070048577 (A1) (also WO2007027730) “Scalable Microbial Fuel Cell with Fluidic and Stacking Capabilities.” This patent publication discloses a three-dimensional anode composed of RVC foam, for example. However, the RVC foam is not made to fill the entire anode compartment, nor is a fluid flow-through scheme utilized.
Therefore, a need exists for increasing the power density of microbial fuel cells. Once the power output of these fuel cells become competitive with other energy technologies, they will be attractive replacements for batteries, for example, in certain application niches. MFCs have the additional advantages of fuel flexibility, being self-regenerating, operating at mild conditions, having high coulombic efficiency, being environmentally benign and intrinsically non-polluting, being robust to fuel interruption, being robust to mixed/impure fuel sources, having no thermal/acoustic signature, and potentially being inexpensive (e.g., unlimited supply of microbes). Each embodiment of the present invention provides a solution to meet such need.